Lignin is a thermo-plastic material that is derived from various biomass materials including hard woods, soft woods and grasses. A thermo-plastic material is a material that when heated to a certain temperature known as the glass transition temperature (Tg) it becomes soft and flowable or extrudable. When the material cools to below the Tg it becomes a solid again. Lignin becomes molten at about 180-200° C. At higher temperatures, lignin decomposes and or reacts with its surrounding environment.
In order to prepare high quality carbon fibers, all of the non-carbon elements in the lignin fibers must be removed from the fibers. Removal of non-carbon elements in the lignin fibers is typically done by heating the fibers to an elevated temperature in an inert gas atmosphere. The foregoing process is called carbonization of the lignin fibers. Prior to carbonization, the lignin fibers are stabilized in a thermal stabilization process. Without stabilization, lignin fibers will soften and possibly form a blob of fused lignin fibers. Hence the thermal stabilization process has to be managed carefully to prevent overheating and or chemical reactions that would damage the fibers. Due to the relatively low glass transition temperature of lignin fibers, the lignin fibers will lose their fiber form if uncontrolled thermal processing is conducted to stabilize the fibers.
Stabilization is a key process to convert lignin fibers to infusible, thermosetting fibers. Thermosetting fibers are fibers that will not soften, as they are reheated or further heated. This normally happens when the precursor polymer (lignin) fibers are oxidation-stabilized in an oxidizing gas (e.g. air) in temperatures of about 300° C. or less, to cause fiber polymers to crosslink. Stabilization of lignin fibers typically starts from a low temperature (usually <Tg) at which an oxidation reaction with the fibers is quite slow. A heating process involving step heating (elevating the fiber temperatures incrementally) is usually used to speed up the stabilization process. Since lignin has a relatively low Tg and melting point, stabilization of lignin fibers is a time consuming process.
During the carbonization of lignin fibers, significantly higher temperatures are used as compared to stabilization temperatures. Accordingly, only stabilized lignin fibers can withstand the significantly higher carbonization temperatures. Without cross-linking or stabilization, the fiber will be degraded during the carbonization thermal processing. The lignin fiber diameter, as spun, is engineered to be very close to the final carbon fiber diameter that is to be produced for specific applications. Even though a certain amount of shrinkage in length and diameter is expected during the stabilization and carbonization process, there are ways to maintain or to control the final fiber diameter. For example, by application of tension during the various thermal processing steps.
During the fiber spinning process of a lignin precursor, the macromolecular chains, to a certain extent, become aligned in the fiber spinning direction (along the fiber axis). However, during the thermal processing (especially when the temperature is higher than the glass transition temperature (Tg) of the lignin fibers), macromolecular chains tend to become free. It is believed that the stabilization process eliminates —OH groups from the fibers. Elimination of —OH groups and reactions in inter- and intra-molecules and subsequent aromatic reactions result in fiber shrinkages. To produce high performance resulting carbon fibers, high orientation of carbon structures (from the lignin molecules) in fiber is sought. Thus, reasonably limiting the fiber shrinkage during thermal processing is desirable.
Carbon Yield (wt %) is another important factor which can affect the cost of carbon fibers. Carbon yield is defined as the ratio of the weight of carbon fibers to the weight of spun lignin fibers. Higher carbon yield is always expected and preferred in carbon fiber manufacturing, as this is an indication of more efficient conversion processing of a given precursor material.
Accordingly, what is needed is a process that can be used to stabilize lignin fibers quickly and obtain higher carbon yield without adversely affecting the length, diameter, and/or strength of the resulting stabilized fibers and carbon fibers.
In one embodiment, the disclosure provides a process for stabilizing lignin fibers comprising heating lignin fibers to a temperature ranging from about 100° to about 220° C. while contacting the fibers with an atmosphere of air bubbled through concentrated hydrochloric acid for a period of time sufficient to stabilize the lignin fibers.
In another embodiment, the disclosure provides stabilized lignin fibers comprised of lignin fibers heated to a temperature ranging from about 100° to about 220° C. while contacting the fibers with an atmosphere of air bubbled through concentrated hydrochloric acid for a period of time sufficient to stabilize the lignin fibers.
In another embodiment, the disclosure provides carbon fibers made by carbonizing stabilized lignin fibers, wherein the stabilized lignin fibers are made by heating the fibers to a temperature ranging from about 100° to about 220° C. while contacting the fibers with an atmosphere of air bubbled through concentrated hydrochloric acid for a period of time sufficient to stabilize the lignin fibers.
Advantages of the disclosed embodiments provide a method for stabilizing lignin fibers in a relatively short period of time while maintaining the diameter and length and increasing the strength of the fibers. The relatively short period of time for fiber stabilization greatly enhances the process of carbonization of the fibers. Compared to conventional lignin fiber stabilization processes, which may take many hours and/or many days, the disclosed embodiments enable fiber stabilization in less than two hours.
Advantages of the disclosed embodiments also provide a method for obtaining higher carbon yield of the resulting carbon fibers while maintaining the diameter and length and the strength of the fibers. Reducing the time for fiber stabilization greatly enhances economics of the process of stabilization and carbonization of the fibers. Compared to conventional lignin fiber stabilization processes, which may result in a carbon yield of about 31.5% for the carbon fiber, the disclosed embodiments enable carbon yield higher than about 40%.
Other features and advantages of the disclosed embodiments may be evident from the following detailed description of exemplary embodiments.